1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device in a horizontal electric field driving scheme.
2. Description of the Related Art
Conventionally, common liquid crystal display devices have been of a type in which an electric field acting in a direction perpendicular to a substrate surface changes orientation of the director (molecular axis) of liquid crystal molecules, thereby controlling transmittance of light to achieve display of an image on a panel (hereinafter referred to as vertical electric field driving type). A TN (Twisted Nematic) mode is a representative mode of the vertical electric field driving type.
In liquid crystal display devices of the vertical electric field driving type, however, the director is oriented perpendicularly to the substrate surface at the application of an electric field. As a result, a refractive index varies depending on a viewing direction causing high dependency upon a viewing angle, so that display devices of this type are inappropriate for applications requiring a wide viewing angle.
To address this, in recent years, research and development have been advanced for liquid crystal display devices of a type in which the director of liquid crystal molecules is oriented parallel to a substrate surface and an electric field acts in a direction parallel to the substrate surface to rotate the director within a plane parallel to the substrate, thereby controlling transmittance of light to achieve display of an image (hereinafter referred to as horizontal electric field driving type). Since liquid crystal display devices of the horizontal electric field driving type exhibit substantially reduced variations in a refractive index depending on a viewing direction, display performance can be obtained with high image quality and a wide field of view.
A prior art active matrix liquid crystal display device of the horizontal electric field driving type will be hereinafter described with reference to FIG. 1 through FIG. 9.
Referring to FIG. 1, there is shown a display pixel which comprises scanning line 502 for connection to an external driving circuit, signal line 103, common electrode 106, thin film transistor 503 serving as a switching element, and pixel electrode 104.
As shown in FIG. 2, on TFT side glass substrate 102, common electrode 106 is formed with pixel electrode 104 and signal line 103 formed thereon interposed by interlayer insulating film 130. At the time of formation, pixel electrode 104 and common electrode 106 are disposed alternately. These electrodes are covered with protective insulating film 110 on which TFT side alignment film 120 required for aligning liquid crystal 107 is applied and subjected to a rubbing treatment. In this manner, TFT side substrate 100 is formed.
On opposite side glass substrate 101, light shield film 203 is provided in matrix form on which color layer 142 required for displaying colors is formed. Additionally provided on color layer 142 is planarization film 202 for planarizing the surface of the opposite side substrate on which opposite side alignment film 122 required for aligning liquid crystal 107 is applied and subjected to the rubbing treatment. The direction of the rubbing treatment is opposite to that of TFT side substrate 100. In this manner, opposite side substrate 200 is formed.
Liquid crystal 107 and spacer 302 are filled between TFT side substrate 100 and opposite side substrate 200. The gap between both substrates is determined by the diameter of spacer 302. Finally, TFT side polarizer 145 is stuck on the surface of TFT side glass substrate 102 which has no electric pattern formed thereon such that the transmission axis thereof is orthogonal to the rubbing direction. Opposite side polarizer 143 is also stuck on the surface of opposite side glass substrate 101 on which no patterns are formed such that the transmission axis thereof is orthogonal to the transmission axis direction of TFT side polarizing sheet 145. With the aforementioned process, liquid crystal display panel 300 is completed.
Thereafter, liquid crystal display panel 300 is disposed above backlight 400 and connected to driving circuit 500 as shown in FIG. 3.
Next, the operation of the liquid crystal display device will be described with reference to FIG. 4 and FIG. 5. FIG. 4 is a circuit diagram showing an equivalent circuit of the prior art liquid crystal display device, while FIG. 5 is a graph showing waveforms of voltages applied to a scanning line, a signal line, and a common electrode, and a waveform of a pixel electrode voltage. It should be noted that Vfd in FIG. 5 is referred to as a feedthrough voltage. The voltage applied to the common electrode is set such that xcex94V+ and xcex94Vxe2x88x92, which represent amplitudes in positive and negative frames of the pixel electrode voltage when an amplitude of a video signal corresponds to a halftone, are equal to each other.
Description is made of the flow of electric charge in a unit element and light switching of the liquid crystal. An ON/OFF signal on scanning line 502 provided in the same layer as common electrode 106 in FIG. 1 causes thin film transistor 503 to switch. When thin film transistor 503 is ON, electric charge flows from signal line 103 into pixel electrode 104. A constant direct-current voltage is always applied to common electrode 106 as described with reference to FIG. 5. In terms of an electric circuit, pixel electrode 104 and common electrode 106 form capacitances CLC, CGL, and CSC across liquid crystal 107, TFT side glass substrate 102, and interlayer insulating film 130, respectively, as shown in FIG. 4.
Thereafter, the charge is held by the capacitances even after thin film transistor 503 turns OFF. The held charge generates a potential difference between pixel electrode 104 and common electrode 106 to create a horizontal electric field parallel to the glass substrate which rotates the director of liquid crystal 107 to change retardation for liquid crystal display panel 300. The changed retardation causes a change in panel transmittance of incident light emitted from backlight 400 shown in FIG. 3 in portions which are not provided with light shield film 203, pixel electrode 104, common electrode 106, scanning line 502, and thin film transistor 503. FIG. 6 shows a relationship between a potential difference between the common electrode and the pixel electrode and the panel transmittance.
The aforementioned prior art liquid crystal display device suffers two disadvantages described below.
A first disadvantage is that the panel transmittance is lowered and uneven display is generated as a charge holding time is reduced. The reason thereof will be described in the following.
Specifically, in the aforementioned prior art liquid crystal display device, although it is desirable that electric charge held by capacitances CLC, CGL, and CSC is completely held when thin film transistor 503 turns OFF, the amount of the charge is actually diminished with a certain time constant in terms of an electric circuit. The time constant xcfx84off is approximately represented as equation (1):
xcfx84off≅Roff(CLC+CGL+CSC+CGS)xe2x80x83xe2x80x83(1)
where Roff represents a resistance of thin film transistor 503 at OFF and CGS represents a gate-source capacitance of thin film transistor 503 shown in FIG. 7.
In a liquid crystal display device of the horizontal electric field scheme, CLC and CGL are smaller than those in a liquid crystal display device of the vertical electric field scheme since they are fringe capacitances. Roff is a constant value determined by a process limit of the thin film transistor and CGS is determined by a size of the thin film transistor, both of which have a low degree of flexibility. Additionally, since CSC corresponds to an overlapping portion between pixel electrode 104 and common electrode 106, an increased area of the overlapping portion leads to a reduced aperture ratio.
When a high light intensity is set for the backlight, a light leakage current is increased resulting from light incident on a back channel of the thin film transistor and the formation of holes in n-i-n parasitic resistance portions as shown in FIG. 7. The leakage of the pixel charge causes a reduction in the potential difference between the common electrode and the pixel electrode, thus lowering the panel transmittance in accordance with the curve in FIG. 6. In addition, since the amount of the light leakage current varies depending on manufacturing variations in the thin film transistor, uneven brightness is likely to occur due to variations in the light leakage current over a display surface.
In addition to the case in which electric charge is leaked through the thin film transistor, when a liquid crystal material with a low resistivity is used, i.e. when a liquid crystal material including multiple ions is used, the ions in the liquid crystal form an electric double layer a while after the writing of charge into the pixel electrode to apparently increase CLC. Since the charge held in the pixel electrode may be considered constant after the thin film transistor turns OFF, the reduction leads to a voltage drop between the pixel electrode and the common electrode. The voltage drop is approximately proportional to the following parameter x referred to as a pixel capacitance ratio:                                           Voltage            ⁢                          xe2x80x83                        ⁢            drop                    ∝          x                =                              C            LC                                              C              GS                        +                          C              LC                        +                          C              GL                        +                          C              SC                                                          (        2        )            
When a sufficiently high CSC can not be set, it is expected that the voltage drop in this model is increased, in which case the panel transmittance is also reduced.
In this manner, an active matrix liquid crystal display device in the horizontal electric field scheme causes a disadvantage that it is difficult to obtain a holding characteristic similar to that of an active matrix liquid crystal display device in the vertical electric field scheme with the aperture ratio maintained, thereby making it difficult to suppress a reduction in the panel transmittance and the generation of uneven display.
A second disadvantage is that enameling and image blotching are generated after long continuous use. The reason thereof will be described in the following.
Specifically, the feedthrough voltage of the thin film transistor is approximately represented as equation (3). Since the denominator in equation (3) is small in a liquid crystal display device of the horizontal electrical scheme, an increased Vfd is exhibited. Additionally, the proportion of CLC in the entire capacitance is higher than that for the vertical electric field scheme. For this reason, when a level of gradation is changed, i.e. when CLC is changed, Vfd varies greatly as compared with the vertical electric field scheme. The increased Vfd means that a difference occurs in Vfd in right and left portions of a display unit when a scanning signal waveform is delayed in a large panel or the like.                               V          fd                =                                            C              GS                                                      C                GS                            +                              C                LC                            +                              C                SC                            +                              C                GL                                              xc3x97                      (                                          V                                  G                  -                  ON                                            -                              V                                  G                  -                  OFF                                                      )                                              (        3        )            
As mentioned above, since a uniform and constant direct-current voltage is applied to the common electrode over the display surface, the aforementioned phenomenon causes increased variations in direct components of the voltage between the pixel electrode and the common electrode both in the surface and between levels of gradation. This results in a problem of the generation of enameling and image blotching due to material deterioration caused by a direct-current voltage applied to a liquid crystal material, and uneven brightness in the display surface due to a difference in effective voltages applied to the liquid crystal in the display surface.
FIG. 8 is a plan view showing a unit pixel in another prior art, while FIG. 9 is a sectional view taken along the b-bxe2x80x2 line in FIG. 8.
The unit pixel shown in FIG. 8 and FIG. 9 is similar to that of the active matrix liquid crystal display device in FIG. 1 and FIG. 2 except that an end portion of pixel electrode 104 is extended to overlap with common electrode 106.
The present invention was made in view of the aforementioned disadvantages, and it is a primary object thereof to provide an active matrix type liquid crystal display device which provides a favorable holding characteristic and a reduced feedthrough voltage, as well as satisfactory display evenness and reliability, with an aperture ratio maintained.
The active matrix type liquid crystal display device according to the present invention has two opposing transparent insulating substrates and liquid crystal interposed between the opposing substrates. On the first substrate, there are provided a plurality of scanning lines and a plurality of signal lines orthogonal to one another, thin film transistors, a common electrodes, pixel electrodes, and a first alignment film.
A thin film transistor is formed near each intersection of a scanning line and a signal line. The common electrodes extend substantially parallel to the scanning lines and each has a plurality of comb-tooth projections extending toward the scanning lines. The pixel electrodes are formed substantially parallel to the comb-tooth projections in the gaps between the adjacent comb-tooth projections of a common electrode when viewing the substrate from the normal direction and a portion of a pixel electrode is opposite to a common electrode interposed by an interlayer insulating film. The first alignment film is formed above the common electrodes interposed by the protective insulating film.
On the second substrate, there are provided a black matrix provided with openings in areas opposite to the pixel electrodes and a second alignment film.
The liquid crystal is controlled by generating an electric field substantially parallel to the layer of liquid crystal with a voltage applied between the pixel electrodes and the common electrodes.
The active matrix type liquid crystal display device according to the present invention further comprises accumulated capacitance increasing means for obtaining an accumulated capacitance between the pixel electrodes and the common electrodes larger than an accumulated capacitance generated in a case where the interlayer insulating film has an even thickness and is of a flat structure.
Additionally, in the present invention, the accumulated capacitance increasing means may be of at least one or more of the following structures:
(1) a structure in which a dielectric with a predetermined permittivity is disposed in the area of the first substrate that are sandwiched between the comb-tooth projections of a common electrode and a pixel electrode when viewing the substrate from the normal direction;
(2) a structure in which a recess is formed in at least one portion of the upper surface area of the first substrate where a common electrode and a pixel electrode overlap when the substrate is viewed from the direction of the normal direction and an interlayer insulating film is interposed between the common electrode and the pixel electrode on a wall surface of the recess;
(3) a structure in which an interlayer insulating film interposed between the common electrodes and the pixel electrodes is formed thinner in at least one portion of an area where a common electrode and a pixel electrode overlap than in other areas; and
(4) a structure in which the interlayer insulating film interposed between the common electrode and the pixel electrodes is formed of a dielectric with a predetermined permittivity in at least one portion of an area where a common electrode and a pixel electrode overlap.
Furthermore, in the present invention, the interlayer insulating film or the dielectric with a predetermined permittivity may be formed from a transparent dielectric preferably titanium oxide having a permittivity higher than the first substrate and the other insulating films formed on the first substrate.
The above and other object, features, and advantages of the present invention will become apparent from the following descriptions based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.